Modulation of quantum yield of primary radical pair formation in photosystem II by site-directed mutagenesis affecting radical cations and anions

Pigment-protein interactions play a significant role in determining the pro perties of photosynthetic complexes. Site-directed mutants of Synechocystis PCC 6803 have been prepared which modify the redox potential of the primar y radical pair anion and cation. In one set of mutants, the environment of P680, the primary electron donor of Photosystem II, has been modified by al tering the residue at D1-His198. It has been proposed that this residue is an axial ligand to the magnesium cation. In the other set, the D1-Gln130 re sidue, which is thought to interact with the C9-keto group of the pheophyti n electron acceptor, has been changed. The effect of these mutations is to alter the free energy of the primary radical pair state, which causes a cha nge in the equilibrium between excited singlet states and radical pair stat es. We show that the free energy of the primary radical pair can be increas ed or decreased by modifications at either the D1-His198 or the D1-Gln130 s ites. This is demonstrated by using three independent measures of quantum y ield and equilibrium constant, which exhibit a quantitative correlation. Th ese data also indicate the presence of a fast nonradiative decay pathway th at competes with primary charge separation. These results emphasize the sen sitivity of the primary processes of PS II, to small changes in the free en ergy of the primary radical pair.